Apparatus for measuring the spatial response of optical systems

ABSTRACT

To measure the spatial response characteristics of optical systems, control circuitry is provided to electrically generate a predetermined one-dimensional spatial waveform (e.g., sinusoidal) on the display screen of a cathode ray tube. The optical system to be tested (e.g., a television system) is placed between the spatial waveform display and a suitable detector which comprises a mask having a narrow slit followed by a photomultiplier. The output of the latter is then displayed on an oscilloscope, or measured in some other known fashion. A linear light modulation is achieved for the spatial waveform display by gating a linearly scanned electron beam with constant amplitude, constant duration, variable duty cycle pulses. Appropriate waveforms can be selected to evaluate the spatial frequency response, transient response, linearity, or steady state response of the optical system under test.

Tlnited States Patent Brown et al.

[151 3,657,550 [4 Apr. 18, 1972 2,731,597 3,193,690 7/1965 Murataetal...

APPARATUS FOR MEASURING THE SPATIAL RESPONSE OF OPTICAL SYSTEMSInventors: Earl Franklin Brown, Piscataway; William Kaminski,l-lunterdon, both of NJ.

Bell Telephone Laboratories, Incorporated, Murray Hill, BerkeleyHeights, NJ.

Filed: Apr. 7, 1970 Appl. No.: 26,292

Assignee:

U.S. Cl. ..250/2l7 CR, 324/20 CR, 178/DlG. 4 Int. Cl ..GOIr 31/22 Fieldof Search ..250/2 1 7 CR; 356/124; 324/20,

324/20 CR; 178/DIG. 4, 5.4 TE

References Cited UNITED STATES PATENTS 1/1956 Schade ..324/20 Sachtleben..356/ 124 Stone ..324/20 Primary Examiner-Walter Stolwein Attorney-R.J. Guenther and E. W. Adams, Jr.

[57] ABSTRACT To measure the spatial response characteristics of opticalsystems, control circuitry is provided to electrically generate apredetermined one-dimensional spatial waveform (e.g., sinusoidal) on thedisplay screen of a cathode ray tube. The optical system to be tested(e.g., a television system) is placed between the spatial waveformdisplay and a suitable detector which comprises a mask having a narrowslit followed by a photomultiplier. The output of the latter is thendisplayed on an oscilloscope, or measured in some other known fashion. Alinear light modulation is achieved for the spatial waveform display bygating a linearly scanned electron beam with constant amplitude,constant duration, variable duty cycle pulses. Appropriate waveforms canbe selected to evaluate the spatial frequency response, transientresponse, linearity, or steady state response of the optical systemunder test.

20 Claims, 9 Drawing Figures SYSTEM Uh l DER TEST CAMERA CRT DISPLAY H24 TRANSMITTINI RECEIVING PHOTO- MCIRCUIT cmcun O DETECTOR I8 I9 21 222a SWEEP GENERATOR/4 CONTROL HORIZONTAL INPUT 0 VERTICAL INPUT CIRCUITRVCR0 PATENTEDAPR 1a 1972 SHEET 3 [IF 6 FIG. 3

PAIENTEDAPR 18 1972 SHEET 5 OF 6 m GD APPARATUS FOR MEASURING THESPATIAL RESPONSE OF OPTICAL SYSTEMS BACKGROUND OF THE INVENTION Thisinvention relates to apparatus used for making objective measurements ofthe spatial response characteristics of optical systems. In aparticularly advantageous embodiment of the invention, the spatialfrequency response of photographic, optical or television systems isreadily measured using such apparatus.

A variety of schemes have been proposed and used for making objectivemeasurements of the spatial response characte'ristics of photographic,optical and television systems; for example, see the bibliographyappended to the article A Method for Measuring the Spatial-FrequencyResponse of a Television System by E. F. Brown, Journal of theS.M.P.T.E., Vol. 76, No. 9, Sept. 1967, pp. 884-888. These techniquesusually employ a target or aperture (e.g., a photographic transparency)which is placed ahead or in front of the system to be measured. Thesystem response to the target or aperture is then recorded on anoscilloscope, photographic film or similar recording device. Usuallysome sort of scanning arrangement is employed such that incrementalmeasurements may be made across the target or aperture surface. Forexample, the photographic transparency is often moved with respect tothe light detector disposed behind the optical system under test; seePhysical Optics in Photography" by G. Franke, The Focal Press 1966), pp.179-185.

The need for a flexible spatial waveform generator to objectivelymeasure the spatial response (e.g., spatial frequency response) ofequipment and components in the optic, photographic and televisionfields has existed for many years (see the aforementioned bibliography).Spatial waveforms are typically obtained by electro-mechanicaltechniques or by photographic gratings. Square wave, photographictransparency, gratings of varying periodicity have proven to be quitepopular This is so because of the difficulty of fabricating other moredesirable spatial gratings such as sinusoidal gratings. As pointed outin the book by Franke, noted above, attempts to make sine wave gratingsby photographic methods have proven inaccurate. As further noted inFranke, pp. 182-183, moire fringe methods have been proposed for thispurpose, as have arrangements for varying the slit length of the mask ofthe light detector means, for disposing the typical square wave gratingon a conical surface, etc. Franke, quite apparently, and others haveexperienced difiiculties with each of these and the otherelectro-mechanical and photographic techniques proposed heretofore. Theuse of a sinusoidal grating to determine the frequency response of anoptical system is particularly advantageous since the same is direct,i.e., the time to frequency transformation typically required by squarewave gratings is not necessary.

Accordingly, it is a primary object of the present invention tofacilitate the objective measurement of the spatial responsecharacteristics of optical systems.

A related object is to generate luminous spatial waveforms havingelectrically controlled periodicity, waveshape and modulation depth.

A more specific object of the invention is to display a sinusoidalspatial waveform of variable periodicity and constant peak luminance ona cathode ray tube.

SUMMARY OF THE INVENTION In accordance with the present inventioncontrol circuitry is provided to electrically generate a predeterminedone-dimensional spatial waveform (e.g., sinusoidal) on the displayscreen of a cathode ray tube. The optical system to be measured (e.g., atelevision system) is placed between the spatial waveform display and asuitable detector which may comprise a mask having a narrow slitfollowed by a photomultiplier. The output of the photomultiplier canthen be displayed on a cathode ray oscilloscope, or measured in someother known fashion. One-dimensional spatial waveforms havingelectrically controlled periodicity, waveshape and modulation depth canbe displayed on the cathode ray tube. Linear light modulation isachieved by means of a half-tone process." The half tone process isachieved by gating a linearly scanned electron beam with constantamplitude, constant duration, variable duty cycle pulses. In thismanner, a light source is obtained whose intensity is linearly relatedto modulating signal. Appropriate waveforms can be rapidly selected toevaluate the spatial frequency response, transient response, linearity,or steady state response of the optical system under test.

It is a feature of the invention to provide a one-dimensional sinusoidalspatial waveform of variable periodicity, which is linearly related inluminosity to an analog control signal.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fullyappreciated from the following detailed description when considered inconnection with the accompanying drawings in which:

FIG. 1 shows a schematic block diagram of apparatus used for makingmeasurements of the spatial response characteristics of a televisionsystem in accordance with the present invention;

FIG. 2 is a schematic block diagram of a spatial waveform generatorconstructed in accordance with the invention;

FIG. 3 shows certain waveforms useful in the explanation of theinvention;

FIGS. 4a, b, 0 illustrate typical waveform patterns that might beobserved on the cathode ray oscilloscope display screen;

FIGS. 5a, b show additional waveforms useful in understanding thepresent invention; and

FIG. 6 is a schematic block diagram of the apparatus arrangementutilized when making measurements of the spatial responsecharacteristics of a lens or lens: system.

DETAILED DESCRIPTION Turning now to FIG. I of the drawings, controlcircuitry 11 is provided to electrically generate a predeterminedonedimensional spatial waveform 12 on the display screen of the cathoderay tube (CRT) 13. As will be explained in detail hereinafter, thedesired waveform is achieved by the application of constant amplitude,constant duration, variable duty cycle pulses to the control grid ofcathode ray tube 13. The sweep generator 14 supplies a typical ramp orsawtooth signal to the horizontal deflection plates of the CRT toprovide a linear scan for the electron beam. As will be later discussed,the sweep generator 14 may be synchronized to a sync signal deliveredfrom control circuitry 11 over lead 15 or, alternatively, the same maybe essentially free-running. In the latter case, the generator 14 de ersa phase comparison signal over lead 16 to the control circuitry 11 for apurpose to be described.

As shown in FIG. 1, the television system under test is placed betweer.the spatial waveform display and a suitable detector. The spatialwaveform 12 observed by the television camera 17 is delivered by thetransmitting circuit 18, along with the necessary televisionsynchronization signals, to the receiving circuit 19 so as to display ontube 21 a replica of wavefonn 12. The lens 22 serves to focus the imageobtained from the spatial waveform replica upon the aperture of mask 23.The lens 22 typically provides some image magnification e.g., 5X). Theaperture in mask 23 may comprise a pinhole, but more often it is anarrow slit (e.g., 0.25 mm in width). The image magnification (5X)provided by lens 22 serves to reduce the effective slit width byone-fifth. The photo-detector 24 is disposed behind the mask 23 and itcan comprise a conventional photomultiplier. The output of thephoto-detector 24 is delivered to the vertical input terminal of thecathode ray oscilloscope 25. A horizontal input signal is also deliveredto the oscilloscope 25 for the purpose of obtaining a stationarydisplay; the latter signal may be derived from the control cir cuitry 11in the manner to be described hereinafter.

The overall test set-up corresponds to that described in theaforementioned Brown article, the significant departure being in thetarget, i.e., the means for generating the luminous display. In theBrown article the target comprises a photographic transparent, theattendant shortcomings of which have been noted above. As will beapparent to those in the art, the use of an oscilloscope is notessential for making measurements of the spatial responsecharacteristics of optical systems, and the photo-detector output can beobjectively measured in accordance with any of the other measurementtechniques known in the art.

FIG. 2 shows a spatial waveform generator, in accordance with theinvention, for producing one-dimensional spatial waveforms ofelectrically controlled periodicity, waveshape and modulation depth.Linear light modulation is achieved by means of a half-tone process.This half-tone process is obtained by gating the linearly scannedelectron beam of cathode ray tube 13 with constant amplitude, constantduration, variable duty cycle pulses. The electrical generation ofspatial waveforms can, of course, also be implemented by intensity orvelocity modulation of the electron beam of the CRT. Unfortunately,however, the trace luminosity is not linearly related to the analogelectrical control signal for intensity or velocity modulation. Eitherof the latter modulation techniques would require that non-linearcorrective networks be used to arrive at a luminous source whoseintensity relates linearly to the amplitude of the electrical controlsignal. Although such a source can perhaps be implemented, it would bevulnerable to relative changes of the CRT transfer characteristicsvis-a-vis the corrective network.

A proportional control arrangement has been devised, in accordance withthe invention, by employing fixed pulse length and amplitude and varyingthe duty cycle thereof to obtain a corresponding average output lightvalue. The space averaged light intensity is proportional to themodulating frequency:

where I,,,,, unmodulated light intensity t /T, instantaneous duty cyclef, instantaneous modulating frequency 1,, pulse width in time The pulsedelectron beam current and spot scanning velocity are constant andlinearity correction is therefore unnecessary.

A test was conducted on a P3l phosphor to show that the luminanceobtained from a trace was independent of the CRT spot packing densitywhen the duty cycle is constant (the latter is achieved by using asquare wave gating or blanking signal). A horizontal time base of 1ms/cm was used and the CRT beam blanking rate was varied from Mb/s to 10Mb/s. The CRT spot density varied from 10 dots/cm to 1,000 dots/cm. Nomeasurable change in luminance occurred for the 100/1 change inmodulating frequency. However, the light emitted by the phosphor wasfound experimentally to be directly proportional to the number ofconstant amplitude, constant duration, variable duty cycle pulsesmodulating the CRT electron beam.

An explanation of the apparatus of FIG. 2 and its operation is perhapsbest served by starting at the CRT grid and working backwards. The CRT13 itself is of conventional design. A monopulser 27 is used to generatea pulse of fixed amplitude and duration which is used to gate (i.e.,turn on) the CRT electron beam. The duration of this gating pulse ismuch smaller than the desired spatial period of the waveform to bedisplayed. The monopulser is triggered by the square wave generator 28at a rate determined by the latter. A typical monopulser output isillustrated by the waveform b of FIG. 3, and the square wave generatortriggering signals are illustrated by waveform c of FIG. 3. As shown inFIG. 3, the monopulser output pulses are coincident with he leadingedges of the square wave triggering signals of FIG. 30; that is, theleading edge of each square wave initiates an output pulse from themonopulser 27. The rate of the square wave generator 28 is voltagecontrolled by an analog signal available from the function generator 29.This analog signal may be either a sine, square, ramp, or triangularwave, or even some processed signal such as an exponential. The spatialwaveform l2 observed on the CRT 13 will be that of the analog signalfrom function generator 29; and the spatial frequency observed is thusdetermined by the frequency of this analog signal, as well as by thesweep speed of the linearly scanned electron beam which is typicallyheld constant.

Considering FIG. 3 in greater detail, the waveform a represents atypical sine wave analog signal derived from function generator 29. Whenthis analog signal is applied to the voltage controlled generator 28,square wave signals are produced having a periodicity or rate whichvaries sinusoidal as shown in FIG. 30. The latter, in turn, trigger themonopulser 27 to produce fixed amplitude, fixed duration pulses whoseduty cycle varies sinusoidally, as illustrated in FIG. 3b. When the sinewave analog signal is of maximum amplitude, (i.e., in and about point31) the monopulser duty cycle is quite high, and the pulses of FIG. 3bare close together. When the analog signal is of minimum amplitude(i.e., in and about point 32) the monopulser duty cycle is quite low andthe pulses thereof are widely separated, as shown in FIG. 3b. Betweenthese two extremes, the aforementioned duty cycle and pulse spacing varysinusoidally.

A monopulser pulse pattern such as shown in FIG. 3b produces aone-dimensional spatial waveform display that varies sinusoidally inluminosity. And the intensity of this luminous display is quite linearlyrelated to the electrical analog signal. When the duty cycle of themonopulser output is high the CRT trace is quite intense, whereas whenthe duty cycle is low the luminosity of the CRT trace is low. Betweenthese extremes the trace luminosity varies sinusoidally in accordancewith the analog control signal from function generator 29. Typically,the function generator will produce a multiple of sine wave signals foreach horizontal sweep or traced produced by the sweep generator 14.Thus, the one-dimensional spatial waveform display will comprise aplurality of spatial sine waves wherein each of the latter variessinusoidally in luminosity in accordance with the modulating analogsignal.

The term one-dimensional is intended to include any spatial distributionof light intensity so long as the intensity varies in one spatialdimensional. In practice, for example, it perhaps is desirable tovertically spot wobble the CRT electron beam so as to provide severalhorizontal lines or traces which vary correspondingly in intensity inone spatial dimension, i.e., horizontally. This affords greater systemsensitivity. Altemately, the CRT may be designed to provide a more orless rectangular aperture for the electron beam.

The output analog signal from the function generator 29 is typically,and preferably, adjustable in frequency, be it a sine, square, ramp, ortriangular wave. However, for any given setting, the output analogsignal is of constant frequency until altered as by changing apotentiometer setting, for example. Depending upon the analog signalwave configuration selected the oscilloscope 25 will display acorresponding pattern such as shown in FIG. 4. This will be more evidenthereinafter. Because the output signal from generator 29 is constant infrequency until altered, as described, the placing of the double-pole,double-throw switch 20 in switch position 1 provides a constantfrequency spatial wave mode of operation. With the switch 20 set toswitch position 2, a frequency modulated spatial wave mode of operationis available. The desirability of providing, in certain instances, alinearly varying spatial frequency display will be readily appreciatedby those in the art.

A linearly varying spatial frequency display can be obtained byfrequency modulating the analog signal of function generator 33 with aramp or sawtooth voltage derived from the function generator 34. Thefrequency variation of the voltage controlled analog function generator33 is determined by the amplitude of the modulating ramp or sawtoothwave. And the frequency of the ramp or sawtooth generator 34 determines,of course, the rate at which the frequency band will be swept.

If the voltage controlled function generator 33 is designed to producesine wave signals and a ramp or sawtooth modulating 5 signal isdelivered thereto from function generator 34, the sine wave output ofgenerator 33 will be frequency modulated in a linear fashion such asshown in FIG. 5a. Each sine wave of the latter figure, in turn, variesthe rate of the square wave generator 28 in the manner illustrated inFIG. 3. The end result is a spatial waveform display such as shown inFIG. 5b. FIG. 5b symbolically illustrates a sinusoidal spatial waveformthat is frequency modulated in a linear manner; such a waveform isparticularly advantageous for making spatial frequency responsemeasurements of optical systems. The varying width of the waveform ofFIG. 5b is intended to denote the varying luminosity of theone-dimensional sinusoidal spatial waveform displayed by the CRT.

As with function generator 29, the analog signal produced by functiongenerator 33 may be either a sine, square, ramp, or triangular wave. Fora linear frequency modulation of the same, the function generator 34delivers a ramp or sawtooth signal to generator 33. However, if thegenerator 34 is designed to provide a sine or triangular wave modulatingsignal, the output wave of generator 33 will be frequency modulated in asinusoidal fashion or in a triangular manner, as the case may be.Function generators of the type described herein are well known in theart; see, for example, the articles Wideband F.M. Generation" by K. K.Clark and D. T. Hess, IEEE Journal of Solid State Circuits, March 1968,pp. 30-31, and Accurate Triangle-Sine Converter by G. Klein, 1967 Digestof Technical Papers, I 967 International Solid-State CircuitsConference, pp. 120l2l. In a test set-up in accordance with theinvention, a WAVETEK General Purpose Voltage Controlled Generator, ModelNo. 1 12, was utilized for each of the generators 28, 29, 33 and 34 ofFIG. 2. This equipment is quite versatile in that it provides a numberof different output waveforms (e.g., sine, square, triangular waves),with selector switches for selecting the one desired.

Equations relating blanking rate and monopulser duty cycle to modulationpercentage, modulation depth and loss introduced by the modulationprocess are given below.

' 1 x F Mod. Depth 10 lo =1olo mdb n min FIA max Light l.oss= 10 log,"10 log db um TI! Where fPK K mr where K modulation sensitivity constantcycles/volts E peak analog modulating signal volts 7 The ratio of thepeak upper and lower frequencies in terms of the center frequencyfi, andpeak deviation is:

As noted in the Brown article supra, some sort of scanning technique istypically employed such that incremental measurements may be made acrossthe target surface. Scanning techniques have been used in which eitherthe target, the optical system itself, or the detector or pickup devicehas been placed in motion. Other scanning techniques involve the use ofmicrodensitometers or similar devices which are scanned acrossphotographic records of the optical system's response.

I In the Brown article, this scanning is achieved by the addition of athird deflection signal to the normal horizontal deflection signalapplied to the camera. The target itself is held stationary. The effectof this third or raster deflection signal is to displace the cameratubes raster which is analogous to moving the target across the camera'sfield of view. This technique of Brown, one of the presentjoint-inventors, can be readily utilized herein. In the arrangement ofFIG. 1, the spatial waveform 12 is held stationary (i.e., no slowdisplacement of drift) in the manner to be described; a rasterdeflection signal is then added to the normal horizontal deflectionsignal applied to camera 17 and it is also delivered to the horizontalinput terminal of the oscilloscope 25. The spatial waveform seen by thephoto-detector apparatus will thus drift slowly with respect thereto andincremental measurements will therefore be made across the entireobserved one-dimensional spatial waveform displayed on CRT 21. Thisdrift of the spatial waveform occurs cyclically and repetitively.

In accordance with the preceding technique, the ramp or sawtooth signalof sweep generator M is phase-locked to the output signal of functiongenerator 29 or 34, as the case may be, so as to achieve a stationaryspatial display on the CRT 13. The output from either of the lattergenerators is applied to sweep generator 14 via switch 20, the ncircuitry 35 and the break contact 36 of a relay (not shown). The breakcontact 36 and make contacts 37 are symbolic of the two techniques ormodes wherein the required spatial waveform scan is achieved, the firstof which has been briefly set-forth above with the other to be describedin detail hereinafter. Since it is usually desirable to display aplurality (n) of waves in the onedimensional spatial waveform display,the sawtooth sweep signal must be at a correspondingly lower repetitionrate. For example, to display 25 sine waves from function generator 29,the sawtooth sweep generator 14 should operate at a rate that isone-twenty-fifth the rate of generator 29. Thus the n circuitry 35 isdesigned to divide by 25 (n=25) for this case. To match the versatilityof the inventive circuitry heretofore described, the n circuitry 35should comprise a plurality of switched divider circuits offeringdivision ratios from 1 to 30, for example. A selector switch associatedwith the n circuit will thus permit the user to select the divisionratio required.

While a square wave signal from either generator 29 or 34 can be divideddirectly, other output signals from the same may require shapingcircuitry which typically is incorporated in the n circuit 35 at a pointpreceding the divider circuit(s). For example, for a sine wave signalfrom] generator 29, a conventional zero-crossing detector should precedethe divide operation. A great variety of shaping circuits, for all sortsof input signal waveforms, are known in the art and the invention is inno way restricted to any particular prior art shaping circuit. And, hereagain, to match the versatility offered by the present invention,several shaping circuits should perhaps be available, any one of whichcan be manually switched into the 3 n/ n circuit depending upon theselected wave configuration from function generator 29 or 34. Aftershaping, if required, and the appropriate division, the n circuit 35delivers a pulse signal to sweep generator 14 to initiate a sweeptherein. This sweep is thus phase-locked to the output of functiongenerator 29 or 34 and the spatial waveform displayed on the CRT 13 istherefore stationary.

In a preferred alternative arrangement for achieving the relativescanning heretofore described, "the sweep generator 14 is notsynchronized but is free-running at a frequency such as to cause a slowdisplacement or drift of the spatial waveform 12 on the CRT display. Tothis end, the break contact 36 is opened and the make contacts; 31closed byactua:

tion of a relay (not shown). Now if the function generator 29, forexample, is operating at a rate or frequency off (e.g., 25 kHz) and nwaves (e.g., 25) are to be displayed on the CRT, then the sweepgenerator 14 should operate at a rate or frequency of (f/n)' '-A, whereA is of the order of 10 cycles per second or less. This offset frequency(A) has the effect of causing the spatial waveform 12 to appear to driftslowly across the CRT display 13. The CRT display 21 drifts in acorresponding manner and incremental measurements are thus made acrossthe entire one-dimensional spatial grating displayed on CRT 21. Theoutput signals of the n circuit 35 and sweep generator 14 are phasecompared in the comparator 38 and a pulse is generated by the latter forevery A cycles of phase difference between the input signals thereto.This pulse is used to trigger the sawtooth generator 39 and the latterthence delivers a sawtooth signal to the horizontal input terminal ofoscilloscope 25 for the purpose of obtaining a stationary display.

Now whether the desired relative scanning is achieved by means of astationary (CRT) spatial waveform and a camera raster deflection, assuggested in the Brown article, or by the preferred technique ofoperating the sweep generator 14 at a non-synchronized offset frequency,the end result is the same, i.e., the one-dimensional spatial waveformdisplayed on CRT 21 is caused to drift slowly with respect to the slitin mask 23. The photo-detector 24 observes the instantaneous lightintensity passed by the slit in each field or frame and it converts thesame to an electrical analog signal sample, which is delivered to theoscilloscope 25.

Portions of typical waveform patterns that may be observed on theoscilloscope are shown in FIG. 4. The three illustrated waveforms arerepresentative of sinusoidal, triangular and square wave spatialgratings of fixed modulating frequency. Time runs from right to left.Each spike or sample occurs at the field or frame rate of the televisionsystem under test. The peak to peak measure (i.e., point 41 to 42) ofthe waveform of FIG. 4a provides an indication of the television systems spatial frequency response at the modulating frequency. Thismodulating frequency can then be increased in steps and a similarmeasure made at each step to arrive at the overall spatial frequencyresponse of the system under test. As the modulating frequency isincreased to higher and higher values, the displayed maximum peak willasymptotically approach a base line level. Alternatively, the linearlymodulated sinusoidal spatial grating of FIG. b will result in anoscilloscope display wherein the envelope of the spikes or samples is afrequency modulated sinusoid, which is distorted by the responsecharacteristics (e.g., frequency dependent loss) of the system undertest. The triangular waveform of FIG. 4b provides an indication of thelinearity response of the system under test and the square waves of FIG.4c indicate the systems transient response.

FIG. 6 shows a test setup for measuring the spatial responsecharacteristics of a lens or lens system. The one-dimensional spatialgrating is developed on the display screen of the CRT 13 in the samemanner as previously described. The sweep generator 14 operates at anon-synchronized offset frequency, as heretofore explained, so as tocause a slow displacement or drift of the displayed waveform 12 withrespect to the slit in mask 23. The photo-detector 24 observes theinstantaneous light intensity passed by the slit for each horizontalscan of the CRT electron beam and it converts the same to an electricalanalog signal sample, which is delivered to the oscilloscope 25. In thepresent case, the CRT electron beam does not scan a complete raster, butrather one or just several horizontal scans or lines are developed.Accordingly, the analog signal samples will occur at a rather highrepetition rate. The low pass filter 61 serves to eliminate these spikesor samples, passing only the envelope of the same to the oscilloscope.The lens or lens system under test can be examined with regard tospatial frequency response, linearity, etc., in much the same manner asthe television system described above.

Although the measuring technique has been described in terms of lightinput to light output, the electrical response, of a television systemfor example, to the input spatial signal may be measured anywherebetween the input and output using standard techniques. For example, asnoted in the Brown article supra, a probe can be connected to thevertical input terminal of the oscilloscope and the electrical responseof a television system may then be measured at any intennediate point inthe same e.g., at the output of the television camera preamplifier.

Particular spectral responses (i.e., selected color response) can beobtained by using selected CRT phosphors and appropriate color filters.Accordingly, it is to be understood that the above-describedarrangements are merely illustrative of the principles of the presentinvention. And other arrangements may be devised by those skilled in theart without departing from the spirit and scope of the invention.

What is claimed is:

1. Apparatus for measuring the spatial response characteristics of anoptical system comprising a cathode ray tube, means for electricallygenerating on the display screen of said cathode ray tube apredetermined spatial waveform whose intensity varies in one-dimensionand is linearly related to the signal electrically generating the same,said generating means including means for varying the periodicity ofsaid spatial waveform, a mask having a small aperture, the opticalsystem under test being disposed between said mask and cathode ray tubeand serving to relay an image of a segment of said spatial waveform tothe aperture in said mask, a photo-detector disposed behind said mask,means coupled to the output of said photo-detector for providing for themeasurement of the light intensity detected by said photo-detector, andmeans for causing a slow scan of the spatial waveform image with respectto said aperture.

2. Apparatus for measuring the spatial response characteristics of anoptical system comprising a cathode ray tube, means for electricallygenerating a one-dimensional spatial waveform on the display screen ofsaid cathode ray tube with the luminosity of said cathode ray displaybearing a linear proportional relationship to the signal electricallygenerating the same, said generating means including means for varyingthe periodicity of said spatial waveform, an opaque mask having a smallaperture therein, the optical system under test being disposed betweensaid mask and cathode ray tube and serving to image a small segment ofthe spatial waveform display upon said aperture, a photo-detectordisposed behind said opaque mask, means coupled to the output of saidphoto-detector for providing for the measurement of the spatial responseof the optical system under test, and means for causing a slow drift ofthe one-dimensional spatial waveform relative to said aperture.

3. Apparatus as defined in claim 2 wherein said electrical generatingmeans serves to generate a one-dimensional sinusoidal spatial waveformof n sine waves.

4. Apparatus as defined in claim 3 wherein said sinusoidal spatialwaveform is frequency modulated.

5. Apparatus as defined in claim 2 wherein said electrical generatingmeans serves to generate a spatial waveform comprising n triangularspatial waves.

6. Apparatus as defined in claim 2 wherein said electrical generatingmeans serves to generate a spatial waveform comie t nsn rst at rq arswayssnw- 7. Apparatus for measuring the spatial response of opticalsystems comprising a cathode ray tube, means for linearly scanning theelectron beam of said cathode ray tube in one given direction, means forgating the linearly scanned electron beam with constant amplitude,constant duration, variable duty cycle pulses so as to generate on thedisplay screen of said cathode ray tube a predetermined one-dimensionalspatial waveform whose luminosity is linearly related to the duty cycleof the gating pulses, means for varying the duty cycle of the gatingpulses in a selected manner, an opaque mask having a narrow slittherein, the optical system under test being disposed between said maskand the cathode ray tube and serving to relay an image of a smallsegment of the spatial waveform display to said slit, a photo-detectordisposed behind said opaque mask, means coupled to the output of saidphoto-detector for providing for the measurement of the light intensitydetected by said photo-detector, and means for causing a slow drift ofthe one-dimensional spatial waveform relative to the slit in said mask.

8. Apparatus as defined in claim 7 wherein the duty cycle of said gatingpulses is varied sinusoidally so as to generate a onedimensionalsinusoidal spatial waveform of n spatial sine waves.

9. Apparatus as defined in claim 8 wherein the sinusoidal variation ofsaid duty cycle is carried out at a linearly varying rate so as toprovide a linearly varying sinusoidal spatial waveform display. 3

10. Apparatus asdefined in claim 7 wherein the duty cycle of the gatingpulses is varied in a manner such as to achieve a spatial waveformdisplay of n triangular spatial waves.

11. Apparatus as defined in claim 7 wherein the duty cycle of the gatingpulses is varied in a manner such as to achieve a spatial waveformdisplay of n spatial square waves.

12. Apparatus as defined in claim 7 wherein the electfon beam scanningmeans is operated at a non-synchronous offset frequency with respect toa sub-multiple of the operating frequency of the duty cycle varyingmeans so as to effect a slow drift in the spatial waveform display.

13. Apparatus for measuring the spatial frequency response row slittherein, the optical system under test being disposed 3 between saidmask and the cathode ray tube and serving to relay an image of a finitesegment of the spatial waveform display tosaid slit, a photo-detectordispo sed b e hi n d opaque mask, oscilloscope means connected to theoutput of said photo-detector for providing a visual indication of theinstantaneous light intensity detected by said photo-detector, and meansfor causing a slow drift in the one-dimensional sinusoidal spatialwaveform displayed on said display screen.

14. Apparatus as defined in claim 13 wherein the sinusoidal variation ofsaid duty cycle is carried out in a linear frequency modulated fashionso as to provide a linear frequency modulated sinusoidal spatialwaveform display.

15. A spatial waveform display for use in making spatial responsemeasurements off optical systems comprising a cathode ray tube, meansfor linearly scanning the electron beam of said cathode ray tube in onegiven direction, means for gating the linearly scanned electron beamwith constant amplitude, constant duration, variable duty cycle pulsesso as to generate on the display screen of said cathode ray tube apredetermined one-dimensional spatial waveform whose luminosity islinearly related to the duty cycle of the gating pulses, means forvarying the duty cycle of the gating pulses in a selected manner, andmeans for causing a slow drift in the spatial waveform generated on thedisplay screen of said cathode ray tube.

16. Apparatus as defined in claim 15 wherein the duty cycle of saidgating pulses is varied sinusoidally so as to generate a one-dimensionalsinusoidal spatial waveform of n spatial sine waves on said displayscreen.

17. Apparatus as defined in claim 16 including means for linearlyvarying the frequency of the sinusoidal variations of said duty cycle soas to provide a linearly varying sinusoidal spatial waveform display.

18. Apparatus as defined in claim 15 wherein the duty cycle of thegating pulses is varied in a manner such as to achieve a spatialwaveform display of n triangular spatial waves.

19. Apparatus as defined in claim 15 wherein the duty cycle of thegating pulses is varied in a manner such as to achieve a spatialwaveform display of n spatial square waves.

20. Apparatus as defined in claim 15 wherein the electron beam scanningmeans is operated at a non-synchronous frequency which is offset withrespect to a sub-multiple of the operating frequency of the duty cyclevarying means so as to effect the slow drift in the spatial w ave formdisplay.

1. Apparatus for measuring the spatial response characteristics of anoptical system comprising a cathode ray tube, means for electricallygenerating on the display screen of said cathode ray tube apredetermined spatial waveform whose intensity varies in one-dimensionand is linearly related to the signal electrically generating the same,said generating means including means for varying the periodicity ofsaid spatial waveform, a mask having a sMall aperture, the opticalsystem under test being disposed between said mask and cathode ray tubeand serving to relay an image of a segment of said spatial waveform tothe aperture in said mask, a photo-detector disposed behind said mask,means coupled to the output of said photo-detector for providing for themeasurement of the light intensity detected by said photodetector, andmeans for causing a slow scan of the spatial waveform image with respectto said aperture.
 2. Apparatus for measuring the spatial responsecharacteristics of an optical system comprising a cathode ray tube,means for electrically generating a one-dimensional spatial waveform onthe display screen of said cathode ray tube with the luminosity of saidcathode ray display bearing a linear proportional relationship to thesignal electrically generating the same, said generating means includingmeans for varying the periodicity of said spatial waveform, an opaquemask having a small aperture therein, the optical system under testbeing disposed between said mask and cathode ray tube and serving toimage a small segment of the spatial waveform display upon saidaperture, a photo-detector disposed behind said opaque mask, meanscoupled to the output of said photo-detector for providing for themeasurement of the spatial response of the optical system under test,and means for causing a slow drift of the one-dimensional spatialwaveform relative to said aperture.
 3. Apparatus as defined in claim 2wherein said electrical generating means serves to generate aone-dimensional sinusoidal spatial waveform of n sine waves. 4.Apparatus as defined in claim 3 wherein said sinusoidal spatial waveformis frequency modulated.
 5. Apparatus as defined in claim 2 wherein saidelectrical generating means serves to generate a spatial waveformcomprising n triangular spatial waves.
 6. Apparatus as defined in claim2 wherein said electrical generating means serves to generate a spatialwaveform comprising n spatial square waves.
 7. Apparatus for measuringthe spatial response of optical systems comprising a cathode ray tube,means for linearly scanning the electron beam of said cathode ray tubein one given direction, means for gating the linearly scanned electronbeam with constant amplitude, constant duration, variable duty cyclepulses so as to generate on the display screen of said cathode ray tubea predetermined one-dimensional spatial waveform whose luminosity islinearly related to the duty cycle of the gating pulses, means forvarying the duty cycle of the gating pulses in a selected manner, anopaque mask having a narrow slit therein, the optical system under testbeing disposed between said mask and the cathode ray tube and serving torelay an image of a small segment of the spatial waveform display tosaid slit, a photo-detector disposed behind said opaque mask, meanscoupled to the output of said photo-detector for providing for themeasurement of the light intensity detected by said photo-detector, andmeans for causing a slow drift of the one-dimensional spatial waveformrelative to the slit in said mask.
 8. Apparatus as defined in claim 7wherein the duty cycle of said gating pulses is varied sinusoidally soas to generate a one-dimensional sinusoidal spatial waveform of nspatial sine waves.
 9. Apparatus as defined in claim 8 wherein thesinusoidal variation of said duty cycle is carried out at a linearlyvarying rate so as to provide a linearly varying sinusoidal spatialwaveform display.
 10. Apparatus as defined in claim 7 wherein the dutycycle of the gating pulses is varied in a manner such as to achieve aspatial waveform display of n triangular spatial waves.
 11. Apparatus asdefined in claim 7 wherein the duty cycle of the gating pulses is variedin a manner such as to achieve a spatial waveform display of n spatialsquare waves.
 12. Apparatus as defined in claim 7 wherein the electronbeam scanning means is operated at a non-synchronous offset frequencywith respect to a sub-multiple of the operating frequency of the dutycycle varying means so as to effect a slow drift in the spatial waveformdisplay.
 13. Apparatus for measuring the spatial frequency response ofan optical system comprising a cathode ray tube, means for linearlyscanning the electron beam of said cathode ray tube in one givendirection, means for gating the linearly scanned electron beam withconstant amplitude, constant duration, variable duty cycle pulses so asto generate on the display screen of said cathode ray tube aone-dimensional spatial waveform whose luminosity is linearly related tothe duty cycle of the gating pulses, means for sinusoidally varying theduty cycle of said gating pulses, an opaque mask having a narrow slittherein, the optical system under test being disposed between said maskand the cathode ray tube and serving to relay an image of a finitesegment of the spatial waveform display to said slit, a photo-detectordisposed behind said opaque mask, oscilloscope means connected to theoutput of said photo-detector for providing a visual indication of theinstantaneous light intensity detected by said photo-detector, and meansfor causing a slow drift in the one-dimensional sinusoidal spatialwaveform displayed on said display screen.
 14. Apparatus as defined inclaim 13 wherein the sinusoidal variation of said duty cycle is carriedout in a linear frequency modulated fashion so as to provide a linearfrequency modulated sinusoidal spatial waveform display.
 15. A spatialwaveform display for use in making spatial response measurements offoptical systems comprising a cathode ray tube, means for linearlyscanning the electron beam of said cathode ray tube in one givendirection, means for gating the linearly scanned electron beam withconstant amplitude, constant duration, variable duty cycle pulses so asto generate on the display screen of said cathode ray tube apredetermined one-dimensional spatial waveform whose luminosity islinearly related to the duty cycle of the gating pulses, means forvarying the duty cycle of the gating pulses in a selected manner, andmeans for causing a slow drift in the spatial waveform generated on thedisplay screen of said cathode ray tube.
 16. Apparatus as defined inclaim 15 wherein the duty cycle of said gating pulses is variedsinusoidally so as to generate a one-dimensional sinusoidal spatialwaveform of n spatial sine waves on said display screen.
 17. Apparatusas defined in claim 16 including means for linearly varying thefrequency of the sinusoidal variations of said duty cycle so as toprovide a linearly varying sinusoidal spatial waveform display. 18.Apparatus as defined in claim 15 wherein the duty cycle of the gatingpulses is varied in a manner such as to achieve a spatial waveformdisplay of n triangular spatial waves.
 19. Apparatus as defined in claim15 wherein the duty cycle of the gating pulses is varied in a mannersuch as to achieve a spatial waveform display of n spatial square waves.20. Apparatus as defined in claim 15 wherein the electron beam scanningmeans is operated at a non-synchronous frequency which is offset withrespect to a sub-multiple of the operating frequency of the duty cyclevarying means so as to effect the slow drift in the spatial waveformdisplay.